"Beat until light" means to use a mixer to beat the ingredients (for example eggs, or eggs and sugar, or butter and sugar) until they are fluffy or frothy. This happens because the beating breaks up fat into tiny globules and causes air to be incorporated into the ingredients. It typically takes 5 to 10 minutes of beating to accomplish this.
"Beat until light" can be done by hand using a wire whisk, but it is really tiring work; still, some cooks insist that whisking by hand produces superior results.